1. Field of the Invention
The invention relates generally to devices which read information from a magnetic medium. More particularly, the invention concerns an improvement in a bandwidth control circuit in a read channel of such a device. The invention also relates to an improved feedback circuit for use in any information channel.
2. Description of the Prior Art
Magnetic recording devices are used to write information to a magnetic medium either in analog or digital form. The magnetic medium is typically a tape, such as used in the well known compact cassette or known high density drives for computers which are used for backup purposes. Another common form is a disc, such as a floppy disc or a hard disc for storing programs and data in a computer.
The read channel for a magnetic recording device includes a sensor in the form of a magneto-resistive head in close proximity with the magnetic medium. When the magnetic material is moved relative to the sensor, a flux is induced in the sensor in dependence of the local orientation of the magnetic material, thereby generating an information signal which can be amplified and then decoded.
When information is written in digital form, such as for computer data storage or digital recording of music, a current is passed through a thin film head in one direction to write a binary xe2x80x9c0xe2x80x9d,and in the opposite direction to write a xe2x80x9c1xe2x80x9d. When the medium is read by the sensor, or read head, the portions recorded with a binary xe2x80x9c0xe2x80x9d will induce a current in the head in the one direction and portions recorded with a binary xe2x80x9c1xe2x80x9d will induce a current in the opposite direction, which is then decoded by a bit detector.
A preamplifier is typically coupled to the sensor, and is the first in a series of blocks that process the analog signal from the sensor to the bit detector. It is desirable for the bandwidth of the preamplifier to be as wide band as possible so that the overall bandwidth of the read channel, from the preamplifier to the bit detector, is much larger than the nyquist frequency of data to maximize recovery of the signal. However, if the bandwidth of the preamplifier is too large, instabilities may occur due to signal and noise artifacts not being filtered away. The preamplifier is typically sold as an integrated circuit to manufacturers of magnetic recording/reading devices, such as hard disk drives. Since the instability problems depend on other components in the read channel selected by the manufacturer, manufacturers often specify that the preamplifier include programmable bandwidth reduction.
Known techniques for controlling bandwidth in an amplifier system include a programmable RC network in the signal path and programmable poles using a transconductance (gm) stage in the forward path of a system. A disadvantage of the first approach is the it loads the signal path during times when it is not desired to reduce the bandwidth, due to the capacitances of the MOS switches which are typically employed to implement a programmable RC network. This is particularly not attractive in high frequency (i.e. high bit rate) systems. A disadvantage of the second approach is that it requires a multiplexor after the programmable pole stage, so as to switch that stage out of the signal path when bandwidth reduction is not required. Multiplexors require additional circuitry, requiring more silicon area.
Accordingly, there is a need for programmable band width control, in devices which read information from a magnetic medium, which does not load the signal path when bandwidth reduction is not needed and/or which does not require extra switching circuitry.
Generally speaking, according to one aspect of the invention, an apparatus with a read channel for reading information from a magnetic medium, includes a read sensor for generating an information signal in response to information stored on a magnetic medium. A gain stage coupled to the read sensor amplifies the information signal from the sensor, the gain stage having an output. A bandwidth control stage includes (i) an emitter-follower having an output, and an input coupled to the output of the gain stage, and (ii) a programmable feedback stage comprising a differentiator with a programmable zero, the feedback stage having an input coupled to the output of the emitter-follower and an output coupled to the output of the gain stage. The location of the zero in the feedback stage controls the bandwidth of the read channel.
One advantage of the above arrangement is that the emitter-follower is in the forward path and has a very wide band width, much wider than the other transconductance (gm) stages. Addition of the bandwidth control circuit, by itself, does not narrow the bandwidth. Another advantage is that the emitter-follower buffers the feedback stage from the gain stage, so that the current drawn by the feedback stage does not effect the operation of the prior gain stage.
According to another aspect of the invention, the programmable bandwidth stage is switchable between an active state, in which the programmable bandwidth stage reduces the bandwidth of the read channel in dependence on a state of the programmed zero, and an inactive state in which the programmable bandwidth stage substantially does not effect the information signal. This is easily implemented with a switch in the feedback stage. The advantage over known bandwidth control circuits is that the forward path is not effected when bandwidth reduction is not required.
According to yet another aspect of the invention, the gain stage includes a load resistor, and the output of the feedback stage is coupled across the load resistor.
The invention also relates to an integrated circuit embodying a preamplifier with bandwidth control.
According to another aspect of the invention, a feedback system includes an emitter-follower in the forward path as a gain stage, and a feedback stage coupling the output of the emitter-follower to the input of the emitter-follower. The feedback system may include switches to selectively switch the feedback stage between (i) the feedback path and (ii) a forward path in series with the emitter-follower. With the switches, a feedback stage which is a differentiator may serve as a differentiator while in the forward path and as an effective integrator while switched in the feedback path. The feedback system using the emitter-follower in the forward path as a gain stage can be used in any information channel.
These and other object, features and advantages of the invention will become apparent with reference to the following detailed description and the drawings.